Vernal sedimentation trends in north Norwegian fjords: temporary anomaly in 234Th particulate fluxes related to Phaeocystis pouchetii proliferation

We report data of a naturally occurring radionuclide, ²³⁴Th, an in situ tracer, to investigate vertical export of biogenic matter during a vernal bloom of Phaeocystis pouchetii in the fjords of northern Norway. To optimise sampling of different stages of the bloom, three fjords with increasing oceanic influence (Balsfjord, Malangen fjord and Ullsfjord, respectively) were investigated in April 1997. Contrasting situations were encountered between the three fjords: the proliferation of P. pouchetii in Ullsfjord surface waters coincided with a drastic reduction of particulate ²³⁴Th fluxes in traps, although particulate organic carbon (POC) and dimethylsulphoniopropionate (DMSP) were exported and ²³⁴Th was available in surface waters. When large colonies make up a significant fraction of the vertical flux, as observed in Ullsfjord in April 1997, there may be a large and rapid change in the POC/²³⁴Th ratio, further complicating the use of ²³⁴Th as a tracer for POC export. The results suggest that the proliferation of Phaeocystis pouchetii during vernal bloom could temporary increase OC/²³⁴Th ratio of particles and delay the particulate export of ²³⁴Th, and probably of other particle-reactive species, from surface waters.